A hybrid approach using Monte-Carlo simulation and an enumeration technique for the reliability evaluation of large scale composite generation-transmission systems is presented. The Monte-Carlo method is based on combining the basic random sampling technique with a direct analytical approach for system analysis and the utilisation of a minimisation model for load curtailment. The method presented is suited to the analysis of large systems and can be used to include multi-state representation of generating units. Studies presented illustrate that derating-adjusted two-state models for generating units can lead to pessimistic appraisals of composite system adequacy. Variability in system load can be incorporated by using a chronological load representation or by an aggregated load model. The accuracy of the load representation and the required computation time increases with the utilisation of more steps in the annual load duration curve. The adequacy indices of different composite systems have different sensitivities to the number of steps in the load curve. A procedure for establishing a set of ratios of generation-transmission adequacy indices which can be used to judge the sensitivities of composite system adequacy indices to the number of steps in the load duration curve is presented. It is only necessary in the case of sensitive composite systems to use many steps in the load model to obtain accurate annual indices and fewer steps can provide sufficiently accurate annual indices in the case of non-sensitive composite systems. A non-uniform load increment step model can effectively improve the accuracy of the calculated annual indices for sensitive composite systems. These conclusions are supported by application of the concepts proposed to two test systems.